Groundwater sapping is erosion by groundwater that emerges as seeps and springs.
Such emergent groundwater has been suggested to be responsible for erosion of the valley
networks on the ancient cratered terrain of Mars as well as other channels on Mars.
Groundwater sapping has been proposed for several reasons. Some of the valley
networks appear to be deeply incised into the intercrater uplands with steep sidewalls and
abrupt terminations in "theater-like" box canyon heads. The apparent
density of channels on the uplands appears to be small, and of uneven spatial
distribution, as compared to water eroded landscapes on Earth. Both of these
characteristics are similar to valleys eroded into sandstones on the Colorado Plateau
which are eroded by emerging groundwater. Another reason that an origin by
groundwater sapping has been proposed is because current models of the early Martian
atmosphere have difficulty maintaining surface temperatures above freezing, even near the
equator. Groundwater sapping is presumed to be able to function at low temperatures
because groundwater could remain unfrozen at depth because of the high geothermal heat
gradient on early Mars, and water emerging at springs might occur beneath a protective ice
cap even at low temperatures. The unfrozen groundwater might be brought close to the
surface by hydrothermal pumping due to water expansion accompanying igneous intrusions or
heat generated by crater impacts. Similarly, upward vapor migration from a deep
water table might resupply shallow groundwater supply. In addition, even if
precipitation occurred at the Martian surface, much of it might have infiltrated downwards
to the the permeable impact-produced "regolith" to emerge in lowlands as
springs.

Initial groundwater sapping simulations examine the case where water infiltrates
underground from surface recharge and emerges in lowlands. A simple model of
groundwater flow assumes that the horizontal scale of flow is much greater than the
vertical scale, and that the subsurface has spatially uniform properties.
Infiltrating water flows laterally and emerges in low areas. This emerging water is
assumed to erode the surface materials by the same process laws that were used for fluvial erosion simulations . Also similar to those
simulations, when erosion creates slopes steeper than the the maximum stable angle for
loose material, downslope slumping is assumed to occur. The first image is the same as the
cratered landscape shown in the crater simulation modeling
except that mass wasting of steep crater walls has occurred. Groundwater is assumed
to infiltrate uniformly over the landscape and to emerge in low areas (primarily at the
bottom of inner crater rims). In the simulation shown here the seepage rates are
assumed to be low enough that no permanent bodies of water form.
After a period of erosion by emerging groundwater, undermining and retreat of deeper
crater rims is apparent. The sediment transported from the zones of emerging groundwater fills in
the lower crater floors. Some short, stubby theater heads are formed at the lower crater
walls. An important characteristic of groundwater sapping is that it essentially
leaves the intercrater plains unmodified. Small craters, in particular, are
unmodified or only slightly eroded. If recharge rates were lower than for the
simulation shown, water would emerge only at the deepest crater floors. If recharge
rates were higher, more and more of the surface would become saturated, and the simulation
would approach that for fluvial erosion.
As with runoff erosion, sapping does not produce much erosion on intercratered plains when
there is no large-scale superimposed relief. The following simulation starts from a
surface in which there is broad-scale relief on which the cratering is superimposed.
This surface also has been subject to eolian deposition. In this
image some mass-wasting has occurred, so that slope gradients on crater inner walls do not
exceed 30 degrees. As with other simulations, the area shown is about 160 x 160 km
with a repeating topographic pattern at 100 km intervals.
After a considerable period of sapping erosion and mass wasting, well-defined sapping
channel systems are formed. Characteristics of sapping channels include:

They form primarily on lowlands. High crater rims and elevated intercrater plains
lack channels.

Crater rims are not dissected except where the upland slopes towards the crater.
In other words, the only channels on crater rims are those emptying into the crater.
(Locations marked with "R"). Channels on outward-sloping crater rims start
some distance from the crater edge (Location marked "T")

Overall drainage density is low.

Valley heads are generally sharp, and valleys are often canyon-like (Locations marked
"C"). However, on some broadly sloping intercrater plains valley heads
feather out (locations marked with "F")

In this and the previous sapping simulation the origin of the groundwater that is
producing the sapping channels is assumed to be areally uniform. This is an
assumption which is most consistent with recharge from surface precipitation. Thus
in these simulations the major difference from the assumptions of runoff erosion is that
water does not directly run off of the surface upon precipitation; rather it infiltrates
and then flows laterally to low areas where it reemerges and instigates channel erosion by
processes identical to those assumed in the runoff erosion simulations. The high
porosity of both eolian deposits and impact crater "regolith" may contribute to
infiltration rather than runoff.

The groundwater sapping simulations are just being started. Further simulations
will examine different initial starting topography, effects of local sources of
groundwater (hydrothermal sources), and situations where some ice-covered surface lakes
might form. In addition, combinations of fluvial and groundwater erosion will be
examined.